1. Field of the Invention
This invention relates to a light emitting diode (LED) based lighting device and in particular to cooling such a device. In particular, although not exclusively, the invention concerns an LED lighting device that can be used as a replacement for a conventional filament lamp such as for example an incandescent light bulb or a halogen reflector lamp. Moreover, the invention concerns an alternating current (AC) driven LED lighting device that can be operated from a high voltage (110/220V) power supply.
2. Description of the Related Art
White light generating LEDs, “white LEDs”, are a relatively recent innovation and offer the potential for a whole new generation of energy efficient lighting systems to come into existence. It is predicted that white LEDs could replace filament (incandescent), fluorescent and compact fluorescent light sources due to their long operating lifetimes, potentially many 100,000 of hours, and their high efficiency in terms of low power consumption. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip or die generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor is combined with the light emitted by the phosphor to provide light which appears to the human eye as being nearly white in color.
To date high brightness white LEDs have been used to replace conventional incandescent light bulbs, halogen reflector lamps and fluorescent lamps. Most lighting devices utilizing LEDs comprise arrangements in which a plurality of LEDs replaces the conventional light source component. For example it is known to replace the filament assembly of an incandescent light bulb with white LEDs or groups of red, green and blue emitting LEDs. WO 2006/104553 teaches such an LED light bulb in which a plurality of white LEDs are mounted on a front face, back face and top edge of a generally rectangular substrate (printed circuit board) such that their combined light emission is generally spherical and replicates the light output of a conventional incandescent light bulb. The substrate is enclosed in a light transmissive cover and mounted to a connector base (e.g. screw cap) for coupling the bulb to a power source. U.S. Pat. No. 6,220,722 and U.S. Pat. No. 6,793,374 disclose an LED lamp (bulb) in which groups of white LEDs are mounted on the planar faces of a polyhedral support having at least four faces (e.g. cubic or tetrahedral). The polyhedral support is connected to a connector base by a heat dissipating column. The whole assembly is enclosed within a transparent bulb (envelope) such that it resembles a conventional incandescent light bulb.
A problem that needs addressing in the development of practical LED lighting devices, in particular compact devices that can be used as direct replacements for incandescent light bulbs, is adequately dissipating the heat generated by the large number of LEDs required in such devices and thereby preventing overheating of the LEDs. Various solutions have been proposed. One solution is to mount the LEDs on a heat sink which comprises the body of the device in which the heat sink is mounted to a conventional connector cap enabling the device to be used in a conventional lighting socket. As for example is described in U.S. Pat. No. 6,982,518 the heat sink can include a plurality of latitudinal fins to increase the surface area of the heat sink. A transparent or translucent domed cover can be provided over the LEDs such that the device bears a resemblance to a conventional light bulb. In U.S. Pat. No. 6,982,518 the form factor of the heat sink is shaped to substantially mimic the outer surface profile of an incandescent light bulb.
In U.S. Pat. No. 6,793,374, to aid in the dissipation of heat, the heat dissipating column can: include a heat sink; include inlet and outlet apertures for aiding air flow within the envelope; be in thermal communication with the cap; or include a fan to generate a flow of air in the lamp.
CA 2 478 001 discloses an LED light bulb in which the LEDs are mounted on a thermally conducting cylindrical core assembly. The core assembly is a segmented structure and comprises a stack of three different disks mounted on a rod. The LEDs are connected to circuit disks that are interposed between insulator disks and metallic disks. The core assembly is enclosed within a diffusing cover that includes an opening in its base and an impeller for creating a uniform turbulent flow of air over the core and out of holes in a cap.
WO 2007/130359 proposes completely or partially filling the shell (envelope) of an LED bulb with a thermally conductive fluid such as water, a mineral oil or a gel. The thermally conductive fluid transfers heat generated by the LEDs to the shell where it is dissipated through radiation and convection as in an incandescent light bulb. Similarly, WO 2007/130358 proposes filling the envelope with a thermally conductive plastic material such as a gel or liquid plastics material.
U.S. Pat. No. 7,144,135 teaches an LED lamp comprising an exterior shell that has the same form factor as a conventional incandescent PAR (parabolic aluminized reflector) type lamp. The lamp includes an optical reflector that is disposed within the shell and that directs the light emitted by one or more LEDs. The optical reflector and shell define a space that is used to channel air to cool the lamp and the LEDs are mounted on a heat sink that is disposed within the space between the shell and the reflector. The shell includes one or more apertures that serve as air inlet and exhaust apertures and a fan is provided within the space to move air over the heat sink and out of the exhaust apertures. Whilst such an arrangement may improve cooling the inclusion of a fan can make it too noisy or expensive for many applications and also less energy efficient due to the electrical power requirement of the fan.
As is known LEDs are intrinsically direct current (DC) devices that will only pass an electrical current in a single direction. In many lighting applications it is desirable to be able to operate LED lighting devices from a high voltage (110/250V) AC mains power supply requiring the use of rectifying circuitry. It is known to house the driver circuitry within the connector cap. It is also known to directly operate LEDs from an AC supply and to eliminate the need for driver circuitry by connecting the LEDs in a self-rectifying configuration. Typically, two strings of series-connected LEDs are connected in parallel with the LEDs in opposite polarity in a half-wave rectifier configuration such that the LEDs are self-rectifying. A sufficient number of LEDs is provided in each string to drop the total source voltage across the LEDs. During the positive half of the AC cycle one string of LEDs is forward biased and energized, while the other string is reverse biased. During the negative half of the AC cycle, the other string of LEDs is forward biased and energized, while the first string is reverse biased and not energized. Thus the strings are alternately energized at the frequency of the AC supply (50-60 Hz) and the device appears to be constantly energized. Although a self-rectifying configuration eliminates the need for separate driver circuitry it has the disadvantage that since only one LED string is energized at a time it has only a 50% payload and is power inefficient. Moreover, concerns have been expressed as to the effect on long term reliability of the LEDs of operating them in a constantly switched mode.
The present embodiments arose in an endeavor to provide an LED lighting device which at least in part overcomes the limitations of the known arrangements and in particular, although not exclusively, addresses the thermal management issues.